Microporous membrane and method for forming

ABSTRACT

The present disclosure describes a method for forming microporous membranes. More specifically, vapor induced phase separation techniques are used for forming multizone microporous membranes having improved material throughput.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a Divisional Application of application Ser. No.13/129,145, filed Oct. 22, 2009, now allowed, which is a national stagefiling under 35 U.S.C. 371 of PCT/US2009/061572, filed Oct. 22, 2009,which claims priority to Provisional Application No. 61/116,841, filedNov. 21, 2008, the disclosure of which is incorporated by reference inits/their entirety herein.

FIELD

The present disclosure relates to a method for forming a microporousmembrane.

BACKGROUND

Microporous membranes with diverse properties are used in many modernproducts, including such things as filters, breathable articles,absorbent articles, and medical articles. There are many known ways tomanufacture microporous membranes, including a phase separation in adope layer. By manipulating the conditions that trigger the phaseseparation, different morphologies can be generated in the resultingmicroporous membrane, adapting it to the specific needs of the end user.

One of the ways that a phase separation can be triggered is bycontacting a dope formulation with a nonsolvent. Methods of makingmicroporous membranes are further described in U.S. Pat. No. 6,736,971(Sale et al.); U.S. Pat. No. 5,869,174 (Wang); U.S. Pat. No. 6,632,850(Hughes et al.); U.S. Pat. No. 4,992,221 (Malon et al.); U.S. Pat. No.6,596,167 (Ji et al.); U.S. Pat. No. 5,510,421 (Dennis et al.); U.S.Pat. No. 5,476,665 (Dennison et al.); and U.S. Patent ApplicationPublication Nos. 2003/0209485; 2004/0084364 (Kools).

Coagulation of dope layers with coagulation baths has been described.Another known method for coagulating dope layers includes introducing anon solvent to the dope layer in the form of a vapor.

SUMMARY

The present disclosure describes a method for forming a microporousmembrane. More specifically, vapor induced phase separation techniquesare used for forming multizone microporous membranes having improvedmaterial throughput.

In one aspect, a method is provided for forming microporous membraneshaving two or more zones (e.g., multizone). The membrane is suitable forhigh material throughput applications. The method includes casting aplurality of dope formulations on a support to provide a multilayersheet having a first major surface, and exposing the multilayer sheet toa first relative humidity level so that water vapor diffuses into thefirst major surface. The method includes exposing the multilayer sheetto a second relative humidity level greater than the first relativehumidity level so that additional water vapor diffuses into themultilayer sheet effecting a phase separation to provide the microporousmembrane. The method also includes washing and drying the microporousmembrane.

In one aspect, a multizone microporous membrane comprising a first zoneand a second zone is described. The multizone microporous membraneindependently comprises pores having average pore diameters, such thatthe average pore diameters of the first zone are greater than theaverage pore diameters of the second zone. The multizone microporousmembrane has a water flux measurement of at least 3,000 lmh/psi and aforward flow bubble point measurement comprising a first zone pressurepeak less than 5 psi and an initial bubble point pressure measurementless than 15 psi.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a process for forming amultizone microporous membrane.

FIG. 2 is schematic representation of a multizone microporous membrane.

FIG. 3 is a schematic representation of a ternary phase diagram.

FIG. 4 is a SEM micrograph (cross-section) of a multizone microporousmembrane of Example 1 having a first zone and a second zone.

FIG. 5 a is a SEM micrograph (planar view) of a first major surface ofthe first zone of the multizone microporous membrane of FIG. 4.

FIG. 5 b is a SEM micrograph (planar view) of a second major surface ofthe second zone of the multizone microporous membrane of FIG. 4.

FIG. 6 is a graphical illustration of a Forward Flow Bubble Point graphof a multizone microporous membrane (FIG. 4) of Example 1.

FIG. 7 is a SEM micrograph (cross-section) of a monozone microporousmembrane of Example 4.

FIG. 8 is a graphical illustration of a Forward Flow Bubble Point graphof a monozone microporous membrane (FIG. 7) of Example 4.

DETAILED DESCRIPTION

Although the present disclosure is herein described in terms of specificembodiments, it will be readily apparent to those skilled in the artthat various modifications, rearrangements, and substitutions can bemade without departing from the spirit of the invention. The scope ofthe present invention is thus only limited by the claims appendedherein.

The term “dope formulation” refers to a composition comprising polymericmaterial and an adjuvant in a solvent.

The term “casting” refers to die forming and depositing dopeformulations in layers to form a multilayered sheet.

The term “relative humidity level” refers to the concentration of watervapor in air and is defined as the ratio of the partial pressure ofwater vapor in the mixture to the saturated vapor pressure of water atthe same temperature. Relative humidity is normally expressed as apercentage.

The term “phase separation” refers to the transformation of a homogenoussystem (e.g., dope formulation) into two or more phases. Examples ofphase separation mechanisms include vapor induced phase separation(VIPS), thermal induced phase separation (TIPS) and liquid-liquid phaseseparation (LIPS).

The term “adjuvant” refers to an additive(s) for a dope formulation.

The term “multizone microporous membrane” refers to a membrane having atleast two distinct porous portions, each of the porous portions referredto as a “zone” or a “microporous zone.”

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.8, 4, and 5).

As included in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. Thus, for example, reference to acomposition containing “a compound” includes a mixture of two or morecompounds. As used in this specification and appended claims, the term“or” is generally employed in its sense including “and/or” unless thecontent clearly dictates otherwise.

Unless otherwise indicated, all numbers expressing quantities oringredients, measurement of properties and so forth used in thespecification and claims are to be understood as being modified in allinstances by the term “about.” Accordingly, unless indicated to thecontrary, the numerical parameters set forth in the foregoingspecification and attached claims are approximations that can varydepending upon the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings of the present disclosure. Atthe very least, each numerical parameter should at least be construed inlight of the number of reported significant digits and by applyingordinary rounding techniques. Notwithstanding that the numerical rangesand parameters setting forth the broad scope of the disclosure areapproximations, the numerical values set forth in the specific examplesare reported as precisely as possible. Any numerical value, however,inherently contains errors necessarily resulting from the standarddeviations found in their respective testing measurements.

The present disclosure uses a VIPS process to form microporousmembranes. The process includes casting dope formulations on a supportto provide a multilayered sheet, and exposing the multilayered sheet towater vapor at two different relative humidity levels. The multilayeredsheet is first exposed to water vapor at a first relative humiditylevel. The water vapor diffuses into the multilayered sheet along thefirst major surface thereof. Not wishing to be bound by theory, exposureto water vapor at a first relative humidity is believed to transform thedope formulations in the multilayered sheet into a metastable state. Themultilayered sheet is then exposed to water vapor at a second relativehumidity level greater than the first relative humidity level. The watervapor at the second relative humidity level also diffuses into themultilayered sheet to increase the concentration of water therein andinduce a phase separation in the dope formulations. Following washingand drying of the water-treated multilayered sheet, each of the originallayers of the sheet become distinct microporous zones which are joinedto one another along a common interface and which, together, form amultizone microporous membrane.

FIG. 1 is a process flow diagram for forming microporous membranes bythe above described method. As shown, dope formulations are first caston a support to form a multilayered sheet. The multilayered sheet isthen exposed to water vapor at a first relative humidity level followedby a second exposure to water vapor at a second relative humidity level.After exposure to the second relative humidity level, the microporousmembrane is typically washed and dried to provide a multizonemicroporous membrane.

FIG. 2 illustrates a multizone microporous membrane 200 according to thepresent disclosure. Multizone microporous membrane 200 has a first zone210 and a second zone 220. First zone 210 and second zone 220 are joinedto one another along a common interface 230. Both the first zone 210 andthe second zone 220 include a plurality of micropores (not shown) formedas a result of the VIPS process. In embodiments of the presentdisclosure, the first zone 210 includes a pore structure with averagepore dimensions that are larger than the average pore dimensions of thepores in the second zone 220. A first major surface 240 of the firstzone 210 is located opposite the common interface 230, and a secondmajor surface 250 of the second zone 220 is located opposite the commoninterface 230 of the multizone microporous membrane 200.

Multizone microporous membranes formed herein are generated without theuse of coagulation baths or from the construction of multiple monozonemembrane layers. The elimination of coagulation baths reduces theoverall costs previously associated with the formation of microporousmembranes by eliminating the need for filtering and cleaning such bathsand related equipment. The resulting multizone microporous membraneshave a combination of high material throughput, fast water flow androbust hydrophilicity.

In the various embodiments, the dope formulations comprise polymericmaterial, an adjuvant, solvent and additives for controlling the rateand depth of phase separation throughout the thickness of a multilayersheet and to influence the formation of a specific microstructure in thefinal multizone microporous membrane.

The concentration of the polymeric materials and/or the adjuvant of thedope formulations can influence the formation of the finalmicrostructure within each of the zones, facilitate the degree ofdiffusion of water vapor into the multilayer at first and secondrelative humidity levels, and influence the integrity of the resultingmicroporous membrane. Simply stated, if the concentration of thepolymeric material of the dope formulation is too low, a membrane willnot be formed. Similarly, if the concentration of the polymeric materialin a dope formulation is too high, an undesired or irregularmicrostructure may result.

The concentration of the polymeric material can be selected, in part, toprovide a desired viscosity and/or surface tension for the dopeformulation in order to facilitate casting of the formulation as a layerin a multilayered sheet. Suitable polymeric materials generally comprisematerials capable of forming micropores (e.g., microstructures) uponexposure to water vapor. In some embodiments, dope formulations includepolymeric materials having a concentration within the range from about 5weight percent to about 15 weight percent based on the total weight ofthe dope formulation. In some embodiments, the concentration of thepolymeric material is in a range from about 7 weight percent to about 14weight percent, or in a range from about 9 weight percent to about 14weight percent based on the total weight of the dope formulation.

In the manufacture of membranes constructed from at least two differentdope formulations, i.e., a first dope formulation and a second dopeformulation, the second dope formulation may have a concentration ofpolymeric material greater than the concentration of polymeric materialin the first dope formulation.

A number of polymeric materials are suitable to include in a dopeformulation, and suitable dope formulations can comprise a singlepolymeric material or a blend of polymeric materials. The polymericmaterials can be amorphous, crystalline, or partially crystalline. Insome embodiments, the polymeric material in a first dope formulation isthe same as the polymeric material in a second dope formulation. Inother embodiments, the polymeric material in a first dope formulation isdifferent than the polymeric material in a second dope formulation.

Examples of suitable polymeric materials include, for example,polyethersulfones, polyetherimides, polyimides, polyamides,polysulfones, polyarylsulphones, polyvinyl chloride, polyethyleneterephthalate, polycarbonates, polyolefins such as polyethylene orpolypropylene, cellulose esters such as cellulose acetate or cellulosenitrate, polystyrenes, acrylic polymers, methacrylic polymers,copolymers of acrylic or methacrylic polymers, and combinations thereof.

In some embodiments, the polymeric material of a dope formulation is apolyethersulfone according to Formula (I).

In further embodiments, the polymeric material of a dope formulation isa polyetherimide according to Formula (II).

In various embodiments, suitable dope formulations are formulated tohave a viscosity that is high enough to permit the formulation to becast as a layer in a multilayered sheet. A suitable viscosity for a dopeformulation may depend on certain process conditions such as the actualor anticipated line speed of a substrate supporting the molten dopeformulations following the casting step. Similarly, factors such assurface tension, general bead stability and other fluid properties ofthe dope formulations are considered for ensuring coating uniformity.The foregoing factors can also affect the diffusion of water vapor intoa dope formulation layer according to the VIPS process utilized herein.

In some embodiments, an appropriate viscosity for a dope formulation iswithin the range from about 2,000 centipoises to about 8,000centipoises. In some embodiments, the viscosity of the dope formulationis in a range from about 2,000 centipoises to about 7,000 centipoises,or in a range from about 3,000 centipoises to about 6,500 centipoises.

The dope formulations herein include at least one solvent. Suitablesolvents are those that dissolve the polymeric material to provide ahomogeneous solution. In various embodiments, the solvent is compatiblewith the polymeric material, adjuvant and any optional additives presentin the dope formulation. The selection of a solvent can be made by oneskilled in the art to influence one or more steps in the VIPS process aswell as the properties of the resulting microporous membrane. Forexample, the selection of a solvent can influence the rate of phaseseparation for the multilayered sheet, the type of microstructure formedin the finished membrane, or the depth of the microstructure formationwithin a layer of the dope formulation. Examples of solvents for dopeformulations useful in the present disclosure include, for example,water, dimethyl formamide (DMF), N,N-dimethylacetamide,N-methyl-2-pyrrolidinone (NMP), tetramethylurea, acetone, methyl ethylketone (MEK), methyl acetate, ethylacetate and other alkyl acetates,dimethylsulfoxide (DMSO), and combinations thereof. In some embodimentsusing a polyethersulfone polymer, the solvent isN-methyl-2-pyrrolidinone. A solvent can be oligomeric or polymeric innature. In some embodiments, the dope formulation can comprise more thanone solvent, or a blend of solvents.

The solvent provides a stable homogeneous solution for casting a dopeformulation to form a microporous membrane. Solvents are categorized as‘good’ solvents, ‘nonsolvents’, and ‘poor’ solvents, depending on theirability to dissolve the selected polymer therein. Solvents categorizedas ‘good’ are those in which the interactions (forces of attraction)between the polymer molecules and solvent molecules are greater than theforces of attraction between polymer molecules. The reverse is true fornonsolvents. Solvents described as ‘poor’ are those in which theinteractions between the polymer and solvent are equal to the forces ofattraction between polymer molecules.

In one embodiment, a stable homogeneous dope formulation can be obtainedby first dissolving the chosen polymer in a good solvent. For dopeformulations having a polymeric material in the form of apolyethersulfone, suitable ‘good’ solvents include, for example,N-methyl-2-pyrrolidinone, dimethylacetamide, dioxane, dimethylsulfoxide,chloroform, tetramethylurea, and tetrachloroethane. In general, goodsolvents are able to dissolve substantial amounts of polymeric material.In some embodiments, a ‘good’ solvent is one that is miscible with thepolymeric material at polymer concentrations of at least about 5 weightpercent based on the total weight of the dope formulation.

One useful method for evaluating solvents for compatibility with apolymer is by the use of Hildebrand solubility parameters. Theseparameters refer to a solubility parameter represented by the squareroot of the cohesive energy density of a material, having units of(pressure)^(1/2), and being equal to(ΔH−RT)^(1/2)V^(1/2)

-   -   where    -   ΔH is the molar vaporization enthalpy of the material,    -   R is the universal gas constant,    -   T is the absolute temperature, and    -   V is the molar volume of the solvent.

Hildebrand solubility parameters are tabulated for solvents in: Barton,A. F. M., “Handbook of Solubility and Other Cohesion Parameters”, 2^(nd)Ed., CRC Press, Boca Raton, Fla. (1991); for monomers and representativepolymers in “Polymer Handbook”, 4^(th) Ed., J. Brandrup & E. H.Immergut, Eds. John Wiley, NY, pp. VII 675-714 (1999); and for manycommercially available polymers in Barton, A. F. M., “Handbook ofPolymer-Liquid Interaction Parameters and Solubility Parameters”, CRCPress, Boca Raton, Fla. (1990).

Adjuvants selected for the dope formulations are generally soluble inthe solvent and are compatible with the polymeric material. Adjuvantscan be added to dope formulations to adjust the viscosity of a dopeformulation prior to casting it as a layer in a multilayer sheet.Similarly, the concentration of adjuvant in a dope formulation caninfluence the diffusion of water vapor into layers of dope formulationsduring the VIPS process. Adjuvants can also be added to dopeformulations to control the rate (kinetics) of the phase separation in aVIPS process. Some useful adjuvants include, for example, poly(alkylene)glycols, polyethers, or combinations thereof. In some embodiments, theadjuvant is poly(ethylene) glycol.

Adjuvants added to dope formulations at selected concentrations caneffect phase separation at a predetermined depth within a layer of adope formulation. In some embodiments, the depth of the phase separationwithin a layer of dope formulation is in a range from about 5 percent toabout 100 percent of the thickness of the layer.

In some embodiments, the concentrations of adjuvant(s) in a plurality ofdope formulations are selected to influence phase separation and providedifferent pore size distributions and different porosities for each ofthe different zones within the multizone microporous membrane. Multizonemicroporous membranes having zones with different average pore diametersare useful in certain high material throughput and high flux filtrationapplications.

In some embodiments, the adjuvant concentration in a dope formulationcan be in a range from about 60 weight percent to 70 weight percentbased on the total weight of the dope formulation. In some embodiments,the concentration of the adjuvant is in a range from about 60 weightpercent to about 68 weight percent, or in a range from about 62 weightpercent to about 68 weight percent based on the total weight of the dopeformulation.

In the manufacture of multizone microporous membranes using a VIPSprocess, the dope formulations are first cast on a support to provide amultilayer sheet. The support can be a plastic or a metal sheet, and itcan be continuous or discontinuous (e.g., discrete). The selectedsupport provides stability for the stacked layers of dope formulationswhile casting and during transport through the first and secondhumidified environments, and during the washing and drying steps.

A plurality of dope formulations is typically cast to form amultilayered sheet on a support. In this configuration, a first dopeformulation layer is stacked onto a second dope formulation layer withan interface formed between the layers. The second dope formulationlayer lays directly on the support and the first dope formulation layeris positioned on top of the second dope formulation, opposite thesupport. The resulting multilayered sheet has a first major surface thatcoincides with the exposed surface on the first dope formulation layer.

In some embodiments, two dope formulation layers are cast simultaneouslyto provide the aforementioned configuration. Simultaneous casting ofmultiple dope formulations can be accomplished using any of a number ofknown techniques and devices. Some useful devices include a multipathapplicator, a dual-knife over roll device, a dual layer slot fed knifedie, and other related devices known in the art for casting dopeformulations.

The thickness of any dope formulation layer is dependent on severalvariables, as known by those skilled in the art. For example, thethickness of a layer can depend on the equipment settings as well as onthe rheology and viscosity of the respective dope formulations.Additionally, the foregoing methods for casting typically involve theuse of a die to shape the dope formulations as they are cast into amultilayered sheet. Consequently, the thicknesses of the dopeformulation layers are directly affected by the gap dimensions of theparticular die slots that are used for the casting operation. Moreover,gap dimensions may be adjusted, in part, to accommodate the viscosity ofa dope formulation. In casting dope formulations using a dual knife overroll device, the gap dimensions can be in a range from about 150micrometers to about 300 micrometers.

After casting the dope formulation layers to form the multilayer sheet,the multilayer sheet is exposed to at least two humidified environmentsto induce phase separation and the formation of microstructures withinthe layered dope formulation layers.

The design of dope formulations that are suitable for formingmicroporous membranes is better understood through a consideration ofprinciples of polymer solubility, the miscibility of components, and theconcentrations of polymer, solvent, and water. At certainconcentrations, polymeric material is completely miscible with solvent.At other concentrations, a region of phase separation exists.

Referring to FIG. 3, a ternary phase diagram 300 is shown for thecomponents of an exemplary dope formulation, i.e., polymerconcentration, water concentration and solvent concentration. FIG. 3illustrates a relationship between the three components. In the diagram300, binodal curve 305 delineates the regions 320 and 330. Each regionrepresents areas of component concentrations for a dope formulation,with region 320 representing a thermodynamically stable concentration ofcomponents and region 330 representing a thermodynamically unstableconcentration of components. Accordingly, the relationship of thepolymer concentration to solvent and water concentrations is delineatedby the binodal curve 305. Region 330 is further divided to include area340 between spinodal curve 310 and binodal curve 305. Spinodal curve 310and binodal curve 305 intersect at point 315 which represents theso-called theta condition (Θ). Not wishing to be bound by theory, at Θ,the interaction forces between the polymeric material molecules and thesolvent molecules equal the interaction forces of polymer molecules forother molecules of the same polymer. A dope formulation having componentconcentrations within area 340 represents a composition believed to bein a metastable state prior to phase separation. A dope formulationhaving component concentrations within area 330 but outside of area 340represents a phase separated composition.

In the processing of a dope formulation via a VIPS process, theformulation is exposed to water vapor to increase the water content ofthe formulation and thereby alter the concentrations of solvent andpolymer. The VIPS process seeks to induce a phase separation of ahomogeneous solution by adding water (e.g., as water vapor) to the dopeformulation and altering the component concentrations of the formulationuntil the component concentrations migrate from region 320, where theformulation is thermodynamically stable (e.g., a solution), to region340 where it is believed to exist in a metastable state. With at leastone additional exposure to water vapor at a second relative humiditylevel, a sufficient amount of water vapor will be diffused into the dopeformulation so that the component concentrations of the formulation moveit out of region 340 and further into the region 330 where phaseseparation occurs.

Humidified environments (i.e., chambers or stations) may be used fordelivering water vapor to the dope formulations of a multilayered sheet.For example, water vapor can be delivered by injecting steam intohumidified chambers. Sensors placed within the chambers can be used tomonitor the actual air temperature and percent relative humidity (e.g.,relative humidity level). Exposure times to water vapor can vary withina useful range depending on factors such as the relative humidity levelsbeing employed, air temperature, and gas phase (e.g., steam) velocity.In various embodiments, the exposure time for the multilayered sheet canbe, for example, in a range from about 7.5 minutes to about 25 minutes.In some embodiments, exposure time can be, for example, in a range fromabout 10 minutes to about 22.5 minutes, in a range from about 10 minutesto about 20 minutes, or in a range from about 12.5 minutes to about 20minutes.

In delivering water vapor to dope formulations comprised of the polymersand solvents described herein, the first relative humidity level can bein a range from about 45 percent to about 55 percent. In someembodiments, the first relative humidity level can be in a range fromabout 46 percent to about 54 percent, in a range from about 46 percentto about 53 percent, or in a range from about 46 percent to about 52percent. After exposure to water vapor at a first humidity level for aperiod of time, the multilayered sheet is exposed to a second relativehumidity level to induce a phase separation. The second relativehumidity level is at least 5 percent greater than the first relativehumidity level. In some embodiments, the second relative humidity levelis at least 6 percent greater, at least 7 percent greater, at least 8percent greater, at least 9 percent greater, or at least 10 percentgreater than the first relative humidity level. The second relativehumidity level can be in a range from about 60 percent to about 80percent. In some embodiments, the second relative humidity level is in arange from about 60 percent to about 75 percent, in a range from about62 percent to about 75 percent, or in a range from about 65 percent toabout 75 percent.

In some embodiments, dope formulations in a multilayered sheet areexposed to water at a relative humidity level intermediate to the firstrelative humidity level and to the second relative humidity level. Theintermediate relative humidity level may be desired for certain dopeformulation or under certain processing conditions in order to graduallyincrease the water content of the dope formulations. In one embodiment,the first relative humidity level can be in a range from about 45percent to 50 percent, the intermediate relative humidity level (e.g.,intermediate) can be in a range from about 50 percent to 55 percent, andthe second relative humidity level can be in a range from about 55percent to 65 percent.

Humidified environments used for the delivery of water vapor and theaforementioned relative humidity levels are typically maintained withina desired temperature range between about 15° C. to about 55° C. In someembodiments, the temperature can be, for example, between about 20° C.to about 50° C., between about 20° C. to about 47° C., or between about20° C. to about 45° C.

Following exposure to water vapor, the resulting phase separatedmicroporous membrane is subjected to washing and drying processes.Washing of the microporous membrane aids in removal of solvents,including water, used in the dope formulations. The washing stepdescribed helps to prevent the microstructure of the membrane fromcollapsing. Washing can be accomplished by spraying, immersing, andother techniques for removing solvents and water. In one embodiment, themembrane is moved through a tank with fluid bearing rollers. Afterwashing, the microporous membrane can be dried by convection, air dryingand vacuum processing. In some embodiments, the membrane is dried atambient conditions in air.

Effective pore sizes (e.g., average pore diameters) formed in themultizone microporous membranes can range from about 0.05 micrometer toabout 2 micrometers. In some embodiments, the pore dimensions from themicroporous membrane can be, for example, in a range of about 0.1micrometer to about 1.5 micrometers, or in a range of about 0.2micrometer to about 0.8 micrometer. In some embodiments, the poredimensions of the microporous membrane can be nearly uniform orsymmetrical through a thickness of one or more of the zones. Zones canhave a symmetrical distribution of pores extending through a portion ofthe thickness of the zone or through the entire thickness of the zone.In some embodiments, multizone microporous membranes formed by themethod described comprise at least two zones having nearly symmetricalpore distributions extending through the thicknesses of their respectivezones.

Pore size of the membranes refers to the average diameter of an openingwithin a microstructure formed during phase separation. Pore sizes canbe measured, for example, by bubble point pressure methods. Some otherpore size and pore size distribution measurement methods can include,for example, solute retention, and flow/pressure techniques. Porediameter can also be estimated by porometry analysis and by a separatemeasurement of the bubble point, with a higher bubble point indicativeof tighter or smaller pores.

Multizone microporous membranes can be formed comprising at least twozones having different pore dimensions within each of their respectivezones.

Microporous membranes formed by the method described herein provide formultizone microporous membranes having a first zone and a second zone.Pores of the first zone provide a first microstructure having largerpore dimensions than the pores formed in the second zone providing asecond microstructure

The microstructures formed can depend on the dope formulation and theprocessing parameters. The first and second microstructures can providea continuous or discontinuous path through the membrane. The formationof the microstructures can depend on the concentration of some of thecomponents (e.g., polymeric material, coating adjuvant, nonsolvent) ofthe dope formulation and the water vapor concentration. The morphology(e.g., symmetric or asymmetric) of the microstructures can furtherdepend on the metering (e.g., layer thickness) of the dope formulations,relative humidity level and/or the rate of phase separation. Themorphology can also depend on the phase separation mechanism, andrelated pressure and temperature processing conditions.

In some embodiments, the first zone of the microporous membrane hasaverage pore dimensions which are greater than the average poredimensions of the second zone. The ratio of the average pore dimensionsof the first zone to the second zone can be, for example, in a range ofabout 10:1 to about 2:1.

Thickness of the microporous membranes formed can be dependent on thethickness of the dope formulation layers when cast, and subsequentremoval of solvents followed by the steps of washing and drying themicroporous membrane. In some embodiments, the thickness of themicroporous membrane can be, for example, in a range of about 125micrometers to about 150 micrometers. In some embodiments, the thicknessof the microporous membrane can be in a range of about 125 micrometersto about 145 micrometers, in a range of about 125 micrometers to about140 micrometers, or in a range of about 125 micrometers to about 135micrometers.

In one embodiment, the thickness of the first zone is greater than thethickness of the second zone of the multizone microporous membrane. Inanother embodiment, the thickness of the first zone is equal to thethickness of the second zone of the multizone microporous membrane.

Multizone microporous membranes formed by the method of the presentapplication can be used in filtration applications. For example, thefirst zone can act as a pre-filter for capturing larger particles andthe second zone can capture smaller particles.

Multizone microporous membranes comprising a first zone and a secondzone comprise pores having average pore diameters. The average porediameters of the first zone are generally greater than the average porediameters of the second zone.

In one aspect, the multizone microporous membrane formed herein has awater flux measurement of at least 3,000 lmh/psi and a forward flowbubble point measurement comprising a first zone pressure peak less than5 psi and an initial bubble point pressure measurement less than 15 psi.

In some embodiments, a combination microporous membrane can be formedcomprising a multizone microporous membrane, as already described,laminated to a monozone microporous membrane. As used herein, the term“monozone microporous membrane” refers to a microporous membrane havingat least one porous zone resulting from the vapor induced phaseseparation of a single dope formulation. In some embodiments, a monozonemembrane can have two or more layers, but the resulting membrane zoneswill have average pore diameters that are substantially the same. Such amonozone membrane is made in the same manner as described herein formultizone microporous membranes but using first and second dopeformulations that are identical or at least are substantially the same.While the monozone membrane includes two ‘zones,’ both zones are of thesame morphology and average pore size and, consequently form a singlefiltration zone.

Lamination of the multizone microporous membrane and the monozonemicroporous membrane can be accomplished using known laminationtechniques including pressure or thermal methods and/or using a suitableadditive or an adhesive. The resulting article is referred to as acombination microporous membrane comprised of a multilayered microporousmembrane having a monozone membrane affixed (e.g., laminated) to themajor surface of the second zone.

Multizone membranes formed herein eliminate the need for combinationmembranes formed by lamination of at least two membranes. Microporousmembranes formed by the described methods can reduce manufacturing costsand increases manufacturing efficiency. The use of humidifiedenvironments to deliver water vapor to sheets for inducing phaseseparation in multilayer sheets eliminates the need for a coagulationbath and multiple washing steps.

The multizone microporous membranes disclosed herein have high materialthroughput. The multizone microporous membranes can be used inpharmaceutical, biological, medical, food and beverage applications. Afilter assembly comprising a cartridge, an inlet, an outlet, and amultizone microporous membrane residing in the cartridge can be used inresidential, commercial and industrial applications.

The disclosure will be further clarified by the following non-limitingexamples.

EXAMPLES

Unless otherwise noted, all parts, percentages, and ratios reported inthe following examples are on a weight basis, and all reagents used inthe examples were obtained, or are available, from the chemicalsuppliers described below, or can be synthesized by conventionaltechniques.

Initial Bubble Point Pressure (IBP)

ASTM Standard E-128-99 (2005). IBP measurements were recorded on 47 mmdiameter pre-wetted microporous membranes with fluorochemical FC-43(Sigma-Aldrich, St. Louis, Mo.).

Water Flow Rate (WFR)

WFR measurements were recorded on the microporous membranes. Themembranes were pre-wetted with isopropanol and deionized water. Thelength of time required for 100 ml of deionized water to pass throughthe microporous membrane under reduced pressure (59 cm Hg) was recorded.The WRF method is further described in U.S. Pat. Nos. 7,125,603 and6,878,419 (Mekela et al.), herein incorporated by reference.

Robust Molasses Throughput (RMT)

RMT measurements were recorded on a multistation stand (e.g. multiplesample station for running several experiments concurrently underidentical process conditions) with a 0.1 wt. % molasses solution (B&GFoods, Incorporated; Parsippany, N.J.). The molasses solution was pumpedat a constant volumetric flow rate of 48 ml/minute through 47 mmdiameter microporous membrane disks. The microporous membranes werepre-wetted with a solution blend of 60 wt. % isopropanol/40 wt. %deionized water. The cumulative volume (ml) of the filtrate was takenwhen the trans-membrane pressure of 25 psi (pounds per square inch) wasachieved.

Forward Flow Bubble Point (FFBP)

FFBP measurements were recorded using 47 mm diameter membranespre-wetted with an isopropanol/deionized water (60/40 vol./vol.)mixture. For the multizone microporous membranes, a first zone pressurepeak and an initial bubble point pressure measurement were recorded.FFBP measurements are similarly described in U.S. Pat. No. 4,341,480(Pall et al.), U.S. Pat. No. 6,413,070 (Meyering et al.), and U.S. Pat.No. 6,994,789 (Sale et al.).

Comparative Examples 1-2 CE1 and CE 2

Commercial two layer membranes having gradient morphologies wereinvestigated: CE1—Sterile High Capacity (SHC) (Millipore, Billerica,Mass.), and CE2—DuraPES TM-600 (Membrana, Wuppertal, Germany).

Example 1

Dope formulations were prepared and delivered to a dual-knife over rolldevice. The first dope formulation (first dope) comprised 9.7 wt. %polyethersulfone (Radel H-2000P; Solvay, Alpharetta, Ga.) dissolved in asolution blend (27.3/63 wt. %) of 1-methyl-2-pyrrolidinone (NMP)(Sigma-Aldrich, St. Louis, Mo.)/polyethylene glycol (PEG-400,(Sigma-Aldrich, St. Louis, Mo.)). The second dope formulation (e.g.,second dope) comprised 14 wt. % polyethersulfone (Sigma-Aldrich, St.Louis, Mo.) dissolved in a solution blend (17/69 wt. %) of1-methyl-2-pyrrolidinone (NMP)/polyethylene glycol (PEG-400).

The first and second dope formulations were co-cast onto a 125micrometer thick polyethylene terephthalate (PET) film (3M Company, St.Paul, Minn.) conveyed at a line speed of 0.41 meters (m)/minute. The gapdimensions of the dual knife over roll device were set at 150micrometers for the second dope formulation and the gap dimension wasset at 225 micrometers for the first dope formulation. The viscositiesof the first dope formulation and the second dope formulation were 3,000centipoises (cps) and 7,500 cps, respectively. The first dope and thesecond dope formulations were cast as layers on one another forming aninterface between the two formulations to provide the multilayer sheet.

The multilayer sheet was introduced into a 7.31 meter longair-floatation dryer line having first and second humidifiedenvironmental chambers, and a washing and drying section. Each of thefirst and second humidified environmental chambers had a length ofapproximately 2.45 m. Steam was injected into the chambers to achievethe first and second relative humidity levels. The relative humidity ofthe humidified chambers was controlled by needle valves placeddownstream of the steam injectors. Humidity sensors were used to monitorthe actual temperature and percent relative humidity in the chambers.The multilayer sheet was exposed to a first relative humidity level of56 percent at 45° C. in a first humidified chamber for water vapor todiffuse into the first major surface. The multilayer sheet was thenexposed to a second relative humidity level of 65 percent at 43.3° C. ina second humidified chamber to effect a phase separation. The resultingarticle was washed and dried to provide a multizone microporousmembrane.

FIG. 4 is an SEM micrograph illustrating, in cross-section, themicroporous structure of the multizone microporous membrane 400according to Example 1. The multizone microporous membrane 400 includestwo individual zones having two distinct pore sizes. The first zone 405has pore sizes of about 0.6 micrometers, and the second zone 410 haspore sizes of about 0.2 micrometers separated by an interface 415. Firstzone 405 of the multizone microporous membrane 400 can provide aprefiltering membrane feature and the second zone 410 can provide asterilizing membrane feature in high throughput filtering applications,for example.

FIG. 5 a is an SEM micrograph (planar view) illustrating a first majorsurface of the first zone 405 of FIG. 4. FIG. 5 b is a SEM micrograph(planar view) illustrating the second major surface of the second zone410 of FIG. 4.

A FFBP curve for Example 1 is illustrated in FIG. 6. The curve supportsa multizone morphology having a first zone and a second zone. In FIG. 6,a first zone pressure peak is exhibited at about 4 psi when nitrogenclears the first zone. The bulk flow at about 11.34 psi indicates thatadequate nitrogen pressure was reached to clear the second zone of themultizone microporous membrane. Test results from Example 1 are shown inTable 1.

Example 2

A multizone microporous membrane was formed in a manner similar toExample 1 with the following exceptions: polyethersulfone polymer(Ultrson E-6020; BASF, location) was used for the first and the seconddope formulations; the first humidity level was 50 percent at 45° C. andthe second humidity level was 65 percent at 43° C. The resulting FFBPprofile (not shown) supported a multizone morphology having a first zonepressure peak of 4.5 psi. Test results from Example 2 are illustrated inTable 1.

Example 3

A multizone microporous membrane was formed in a manner similar toExample 1 with the following exceptions: the gap dimension of the dualknife over roll device for delivering the first dope layer was set at350 micrometers; the gap dimension for delivering the second dope layerwas set at 125 micrometers; the first relative humidity level was 48percent at 47.2° C.; the second relative humidity level was 70 percentat 45.6° C. The resulting FFBP profile (not shown) supported a multizonemorphology having a first zone pressure peak of 4.5 psi. Test resultsfrom Example 3 are illustrated in Table 1.

Example 4

A single-layer monozone microporous membrane (sterilizing-grademembrane) was prepared for use in the construction of a combinationmembrane. The monozone microporous membrane was formed using the dualknife over roll device of Example 1. The second dope formulation ofExample 2 was cast and exposed to a first relative humidity level of 43percent and 45° C. The single dope formulation layer was then exposed toa second relative humidity level of 65 percent and 43.3° C. Theresulting material was washed and dried.

FIG. 7 is an SEM micrograph of a cross-section of the monozonemicroporous membrane, showing a symmetrical morphology through theentire thickness of the monozone microporous membrane.

A FFBP curve for Example 4 is illustrated in FIG. 8. The curve supportsa monozone morphology. In FIG. 8, nitrogen (g) clears the single zone ata peak of about 35 psi. The monozone microporous membrane was tested,and the results are illustrated in Table 1.

Monozone microporous membranes of FIG. 7 can be applied (e.g.,laminated) to multizone microporous membranes for forming combinationmicroporous membranes. The combination microporous membrane can have themonozone membrane (first layer) as a sterilizing membrane and amultizone microporous membrane (second layer) functioning as apre-filtering membrane.

TABLE 1 Turbidity Water *RMT Reduction Flux IBP Sample (ml) (%)(lmh/psi) (psi) 1 2969 0.55 3450 11.34 2 2950 0.65 5036 12.4 3 4000 0.357456 7.7 CE 1 1500 0.74 2500 12.5 CE 2 2500 0.74 2500 21 4 100-300 0.9N/A 35 *Robust Molasses Throughput

Various modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this disclosure, and it should be understood that thisdisclosure is not limited to the illustrative elements set forth herein.

What is claimed is:
 1. A multizone microporous membrane comprising afirst zone and a second zone independently comprising pores havingaverage pore diameters, such that the average pore diameters of thefirst zone are greater than the average pore diameters of the secondzone, the multizone microporous membrane having a water flux measurementof at least 3,000 lmh/psi and a forward flow bubble point measurementcomprising a first zone pressure peak less than 5 psi and an initialbubble point pressure measurement less than 15 psi.
 2. The multizonemicroporous membrane of claim 1, wherein the first zone has a firstthickness and the second zone has a second thickness, the firstthickness being greater than the second thickness.
 3. The multizonemicroporous membrane of claim 1, wherein the first zone has average porediameters in a range from about 0.5 micrometers to about 0.7micrometers, and the second zone has average pore diameters in a rangefrom about 0.1 micrometers to about 0.3 micrometers.
 4. The multizonemicroporous membrane of claim 1, wherein the first zone and the secondzone independently have a symmetrical morphology.
 5. The multizonemicroporous membrane of claim 1, wherein the first zone comprises apolymeric material selected from the group consisting ofpolyethersulfones, polyetherimides, nylons, polyimides, polyamides,polysulfones, polyarylsulphones, polyvinyl chloride, polyalkyleneterephthalates, polycarbonates, polyolefins, cellulosics, polystyrenes,acrylic polymers, methacrylic polymers, copolymers of acrylic ormethacrylic polymers, and combinations thereof.
 6. The multizonemicroporous membrane of claim 1, wherein the second zone comprises apolymeric material selected from the group consisting ofpolyethersulfones, polyetherimides, nylons, polyimides, polyamides,polysulfones, polyarylsulphones, polyvinyl chloride, polyalkyleneterephthalates, polycarbonates, polyolefins, cellulosics, polystyrenes,acrylic polymers, methacrylic polymers, copolymers of acrylic ormethacrylic polymers, and combinations thereof.
 7. The multizonemicroporous membrane of claim 1, wherein the first zone and the secondzone comprise the same polymeric material.
 8. The multizone microporousmembrane of claim 1, wherein the first zone comprises a polyethersulfoneaccording to Formula (I):


9. The multizone microporous membrane of claim 1, wherein the first zonecomprises a polyetherimide according to Formula (II):


10. The multizone microporous membrane of claim 1, wherein the ratio ofthe average pore diameters of the first zone to the second zone are in arange of about 10:1 to about 2:1.
 11. A combination microporous membranecomprising the multizone microporous membrane of claim 1 laminated to amonozone microporous membrane, the monozone microporous membranelaminated adjacent to the second zone, wherein the average porediameters of the monozone microporous membrane are smaller than theaverage pore diameters of the second zone.
 12. A filter assemblycomprising a cartridge having an inlet and an outlet, and the multizonemicroporous membrane of claim 1 residing within the cartridge.